The Invisible Living World:

Beyond Our Naked Eye

NCERT Class 8 Science  |  Chapter 2

The human eye can see objects only above a certain size. For a long time, countless tiny things around us remained hidden and unknown. Long ago, people discovered that a curved piece of glass — shaped like a lentil seed, thick in the middle and thin at the edges — could make small things look larger. They called it a lens. Over time, lenses became more powerful, eventually giving birth to the microscope. The invention of the microscope unlocked a fascinating hidden world filled with tiny living creatures. All living things — whether plants or animals — are called organisms. A water-filled round-bottom flask placed over text magnifies the letters just like a lens does, demonstrating that curved, transparent objects bend light and enlarge images.

  Ever Heard Of … Robert Hooke & Leeuwenhoek  

In 1665, Robert Hooke published Micrographia — the first book with detailed microscopic drawings. Using a microscope that magnified objects 200–300 times, he examined a thin slice of cork and noticed it was made of tiny, empty, honeycomb-like compartments. He called each compartment a cell — the first time this word was used in science to describe the basic unit of life.

Around the same time, Antonie van Leeuwenhoek, a Dutch scientist, built far superior microscopes using better lenses. He became the first person to clearly see and describe living microorganisms like bacteria and blood cells — earning him the title 'Father of Microbiology'.

2.1 What Is a Cell?

All living beings are made up of cells — the basic building blocks of life. When you observe the thin, transparent layer peeled from an onion bulb under a microscope (stained with safranin to improve visibility), you see nearly rectangular structures packed tightly together with no space between them. These are onion peel cells. Similarly, scraping the inner lining of your cheek and viewing it under a microscope reveals polygon-shaped cheek cells. Both types of cells reveal the same fundamental structure.

Every cell has three main parts:

• Cell membrane — The outermost layer of the cell (in animal cells). It encloses the cytoplasm and nucleus, separates one cell from another, and is porous — allowing essential materials to enter and waste to exit.

• Cytoplasm — The jelly-like space between the cell membrane and the nucleus. It contains carbohydrates, proteins, fats, and mineral salts. Most life processes occur here.

• Nucleus — The round, central structure covered by a thin membrane. It acts as the control centre of the cell — regulating all cell activities and growth.

Some cells, like onion peel cells (plant cells), have an additional outer layer called the cell wall, which provides rigidity and strength to the plant — just like a brick wall gives strength to a building.

  A Step Further: More Structures in Plant Cells  

Plant cells have additional structures not found in most animal cells. Tiny rod-shaped structures called plastids are found in all plant cells. A special type called chloroplasts contains chlorophyll — the green pigment responsible for photosynthesis. Plant cells also contain a large, prominent vacuole — an empty-looking space that stores important substances, removes waste, and maintains the cell's shape and support. In animal cells, vacuoles are either absent or very small. Both plant and animal cells also contain mitochondria, the energy-producing structures of the cell.

A cell is not a simple bag of liquid — it is a complex, organised structure where each part has a specific, vital function.

2.1.1 Variation in shape and structure of cells

Cells are not all the same — they vary dramatically in shape and size, and this variation is directly linked to their function.

• Cheek cells — Thin and flat; form a smooth protective lining inside the mouth.

• Nerve cells (neurons) — Very long with multiple branches; this elongated, branched structure allows them to transmit messages rapidly across different parts of the body.

• Muscle cells — Spindle-shaped, thin, and flexible; this shape allows them to contract and relax in a wave-like manner, pushing food down the food pipe and churning it in the stomach.

• Plant cells — May be rectangular, elongated, oval, or tube-like. Some form long tubes to transport water throughout the plant.

The digestive system is a perfect example of how specialised cells work together. Muscle cells in the food pipe push food downward; muscle cells in the stomach churn it; and other cells in the stomach lining secrete digestive juices and acids. Each type of cell is shaped exactly for its job.

2.2 What Are the Levels of Organisation in the Body of a Living Organism?

Life is organised in a beautifully hierarchical way — from the tiniest building block to the complete organism. Just as individual bricks come together to form a wall, individual cells come together to form a living body.

The levels of organisation are:

Cell → Tissue → Organ → Organ System → Organism

• Cell — The basic unit of life (e.g., muscle cell, nerve cell).

• Tissue — A group of similar cells working together (e.g., muscle tissue).

• Organ — Different tissues organised to perform a specific function (e.g., stomach).

• Organ System — Several organs working together to carry out a major body function (e.g., the digestive system).

• Organism — All organ systems together forming a complete living being (e.g., a human, a plant).

The life of complex organisms begins with a single cell — the egg. This single cell divides repeatedly to form a complete being made of billions of cells. Such organisms are called multicellular organisms. Animals, including humans, and all plants are multicellular.

  Ever Heard Of … The World's Largest Cell  

The yolk (yellow part) of an ostrich egg is a single cell — the largest known cell in the living world, measuring about 130 to 170 mm in diameter! The surrounding shell and egg white are extra non-cellular materials that protect and nourish this giant cell during development.

2.3 What Are Microorganisms?

Some living organisms are made up of just one or very few cells. They are so tiny that they cannot be seen with the naked eye — these are called microorganisms or microbes (micro = very small; organism = living being). They are found everywhere — in water, soil, air, and even inside our bodies.

Microorganisms can be:

• Unicellular — Made of just one cell (e.g., Amoeba, bacteria).

• Multicellular — Made of many cells (e.g., some fungi and algae).

To see microorganisms, we use a microscope — a device that magnifies them 100 to 400 times. Scientists have also developed affordable foldable paper microscopes that make the microscopic world accessible to more people. Here are the main types of microorganisms found in pond water and soil:

  Ever Heard Of … Viruses  

Viruses are microscopic and acellular — they are not made of cells. Unlike other microorganisms, viruses can only multiply by entering a living cell (called a host). They can infect plants, animals, and even bacteria, and often cause diseases.

2.4 How Are We Connected to Microbes?

Microorganisms are everywhere around us — in water, soil, air, on food, on surfaces, and even inside our bodies. When a lemon or orange is left out for too long, you may notice a powdery or cotton-like growth forming on it. This is caused by microbes that have colonised the food from the environment. Microbes show great diversity in shape, size, and structure — some thrive in extreme conditions like hot springs and freezing cold zones.

Some microbes already live inside us! Our intestine contains bacteria that help in digestion. And then there are microbes that decompose, ferment, and even enrich the soil. Let's explore their many roles.

Why don't pickles and murabbas get infected? The high concentration of salt or sugar acts as a preservative — it prevents microorganisms from growing, keeping the food safe for a long time.

2.4.1 Key players in cleaning the environment

When fruit and vegetable peels are buried in garden soil and left for 2–3 weeks, they gradually transform into a dark, nutrient-rich material called manure. This happens because microorganisms — especially bacteria and fungi — present in the soil act on the organic waste and break it down into simpler, nutrient-rich substances. This process is called decomposition.

Decomposition is nature's recycling system. Fallen leaves, dead plants, and animal bodies are all broken down by microorganisms, and their nutrients are returned to the soil — helping new plants grow. Manure formation works best at optimal temperature and moisture levels. Helpful bacteria can even decompose animal dung.

  Our Scientific Heritage  

Ancient Indian texts, particularly the Vedas, mention the word 'Krimi' — referring to tiny entities, both Drishya (visible) and Adrishya (invisible). Various Vedic texts, including the Atharvaveda, describe both the beneficial and harmful effects of these tiny organisms — indicating an early awareness of the microbial world thousands of years before the microscope.

  A Step Further: Microbes as a Source of Biogas  

Some bacteria and fungi that live in oxygen-free environments can decompose plant and animal waste, releasing a mixture of gases called biogas — primarily methane and carbon dioxide. Biogas is an eco-friendly fuel used for cooking, heating, generating electricity, and even running vehicles. India has a long history of biogas production, with one of its oldest plants set up in the 1850s. The Government of India's Biogas Programme under the Ministry of New and Renewable Energy continues to promote this sustainable technology.

  Be a Scientist: Ananda Mohan Chakrabarty  

Ananda Mohan Chakrabarty (1938–2020) was an Indian-American scientist who studied bacteria. In 1971, he genetically engineered a bacterium capable of breaking down crude oil spills, offering a microbial solution to environmental pollution. His invention received a patent in 1980 — a legal protection that prevents others from copying, using, or selling someone's invention without permission. His work demonstrated the immense potential of microorganisms in solving real-world environmental problems.

2.4.2 Microorganisms and food

Microorganisms play a key role in the preparation of many everyday foods through a process called fermentation.

Yeast and bread-making: Yeast is a type of microorganism belonging to the fungi group. It grows well in warm conditions. When yeast is added to flour dough with a little sugar and warm water, it breaks down the sugar and releases carbon dioxide gas. The gas forms bubbles that make the dough rise, becoming soft and fluffy — exactly how breads and cakes are made! A small amount of alcohol is also produced, giving the dough a slightly different smell.

Lactobacillus and curd: The bacterium Lactobacillus is responsible for turning milk into curd. It feeds on the sugar in milk (lactose), multiplies, and produces lactic acid — which makes the curd sour. This process works best in warm conditions; in cold temperatures, curd does not form properly. The same principle applies in food factories for large-scale fermentation.

Other bacteria, including Lactobacillus, also help ferment batter for making idli, dosa, and dough for bhatura.

Rhizobium and soil fertility: Some bacteria, like Rhizobium, live in swollen structures called root nodules on the roots of legumes like beans, peas, and lentils. These bacteria fix atmospheric nitrogen — converting it into a form plants can use. This reduces the need for chemical fertilisers and keeps the soil naturally fertile. That is why farmers practice crop rotation — alternating legume crops with others to naturally enrich the soil.

2.4.3 Amazing microalgae: tiny helpers in water

Microalgae are microscopic plant-like organisms that live in water, soil, air, and even on trees. They produce their own food through photosynthesis and, in doing so, release oxygen — contributing to more than half of the Earth's total oxygen supply. They are rich in nutrients and serve as a food source for countless aquatic animals.

Some species of microalgae have direct human benefits:

• Spirulina — A protein-rich superfood (over 60% protein by body weight); also a source of Vitamin B12. Used as a health supplement and can be farmed in glass tanks.

• Chlorella — Used as a nutritional supplement and for water purification.

• Diatoms — Used in medicines and industrial applications.

Microalgae also help in cleaning water and are used to produce biofuel. However, pollution, climate change, and habitat destruction are threatening microalgal populations. Conserving microalgae is important for maintaining the Earth's oxygen balance and ensuring food and livelihood security — including through Spirulina farming as a sustainable livelihood option.

2.5 Why Is Cell Considered to Be a Basic Unit of Life?

Every living organism — from a giant whale to the tiniest bacterium — is made up of cells. A single cell contains all the components needed to carry out the functions essential for life. This is why the cell is called the basic unit of life.

Organisms can be classified based on how many cells they have:

• Unicellular organisms — Made of just one cell that performs all life functions (e.g., bacteria, Amoeba, yeast).

• Multicellular organisms — Made of many cells, each carrying out specialised functions while cooperating with others (e.g., plants, animals, humans, mould).

While plant, animal, and fungal cells share the basic structures (cell membrane, cytoplasm, nucleus), there are important differences:

• Bacterial cells — Do not have a well-defined nucleus or nuclear membrane. Instead, they have a region called the nucleoid where their genetic material is located. This is the key distinction between bacteria and all other cells.

• Fungal cells — Have a cell wall but lack chloroplasts, so they cannot make their own food through photosynthesis.

• Plant cells — Have a cell wall, chloroplasts, and a large vacuole.

• Animal cells — Have only a cell membrane (no cell wall), and vacuoles are absent or very small.

For observing internal components of cells in even greater detail, scientists use electron microscopes — which can magnify objects up to 10,00,000 times. Such microscopes reveal structures that are far too small for even the best light microscopes.

In this chapter, we have learned about the beneficial roles of microorganisms. However, some microbes also cause diseases in plants, animals, and humans — a topic explored in the next chapter.

  Quick Recap: Snapshots  

• Microorganisms are tiny living beings not visible to the naked eye; they are found in water, soil, air, and inside living organisms.

• They can be unicellular (bacteria, protozoa) or multicellular (fungi, algae). Plants and animals are always multicellular.

• The cell is the basic unit of life. All organisms — including microbes — are made of one or more cells.

• A typical cell has three main parts: cell membrane, cytoplasm, and nucleus. Plant, fungal, and bacterial cells also have a cell wall. Bacteria lack a well-defined nucleus (they have a nucleoid instead).

• Cells differ in shape and size — and their shape is always related to the function they perform.

• Microorganisms perform decomposition — breaking down organic waste into nutrients that go back to the soil.

• Rhizobium bacteria in root nodules of legumes fix atmospheric nitrogen, naturally fertilising the soil.

• Yeast (a fungus) is used to make breads, cakes, idli, dosa, and bhatura through fermentation.

• Lactobacillus bacteria convert milk into curd by producing lactic acid.

• Viruses are microscopic and acellular — they can only reproduce inside a host cell.

• Microalgae produce more than half of Earth's oxygen and are used as health supplements, biofuels, and water purifiers.

• Microorganisms can be beneficial or harmful — some cause diseases while others clean the environment and enrich soil.